Extensive studies have shown that an aggressive mucosal T cell subset that reacts to intestinal bacteria plays an important role in the cause of inflammatory bowel disease (IBD). These aggressive T cells are thought to live long as immune-memory cells in peripheral blood and contribute to the chronicity and the relapse of the disease. Evidence further suggests that a type of T cells, which flexibly secretes different effector molecules, contributes to the complex pathogenesis of IBD. Another T cell subset, T follicular helper (Tfh)-like cells, causes severe colitis in animal models, and is abundantly found in peripheral blood of IBD patients. Because Tfh-like cells are immune-memory cells with highly flexible secretion of effector molecules and are generated in the intestine, they may contribute significantly to chronic inflammation in IBD. However, the role of Tfh-like cells in intestinal inflammation and the regulatory mechanisms of their function in the intestine have yet to be explored. The objective of this application is to identify the key pathways regulating Tfh-like cell generation and function in the intestine. We have demonstrated that TIR-domain-containing adapter-inducing interferon-? (TRIF), a molecule that is activated by encountering bacteria and viruses, plays an important role in maintaining intestinal homeostasis through direction of T cell activation and function. We have found that TRIF limits the generation of Tfh-like cells and T cell flexibility of effector molecule secretion in the intestine through the pathways induced by four key molecules; IFNAR, IL27, STAT1, and STAT3. Impairment of TRIF amplifies Tfh-like cell generation and plasticity, and increased levels of TRIF results in suppression of Tfh-like number and flexibility. Genetic studies have identified that the abnormalities in the IFNAR, IL27, STAT1, and STAT3 genes are often found in Crohn's disease (CD) patients. Therefore, through the following three aims, we would like to address our hypothesis that impairment of the TRIF-mediated pathways allows increased generation of functionally flexible Tfh-like cells that contribute to chronic inflammation in CD, particularly in patients with genetic variants in the IL27, IFNAR, STAT1, and STAT3 genes. Our specific aims are: (1). Determine the mechanism by which Tfh-like cells perpetuate destructive inflammation in the intestine. (2). Determine the mechanisms by which TRIF regulates pathogenic Tfh-like cell generation. (3). Determine the functional consequence of genetic variants within TRIF-mediated pathways in the generation of pathogenic Tfh-like cells in patients with CD. Understanding target pathways affecting the generation of highly aggressive T cells may suggest novel approaches, such as precision medicine, in the management of the subsets of IBD patients in which inflammation is caused by these T cells. The outcome of this project will be novel insight into the essential regulatory mechanisms of highly aggressive T cell populations, which is expected to have a major impact in our understanding of mucosal immunology, as well as in the future therapy of many chronic inflammatory diseases including IBD.